It is known that, given a pair of stereoscopic photograms of an object photographed with a camera arranged at two positions and, within appropriate limits oriented in any manner, a stereocomparator is that kind of photogrammetric instrument which allows measuring the plate coordinates (x.sub.1 y.sub.1 on the first photogram, and x.sub.2 y.sub.2 on the second photogram) which define, with respect to a system of orthogonal axes defined on the same photograms, the position of two images I.sub.1 and I.sub.2 corresponding to the same point P of the photographed object.
Stereocomparators furthermore include support and illumination for two photograms and two microscopes provided with a reticle or the like for binocular collimation. Members providing for relative movement are also employed between the two photograms and the two microscopes run by means of movable sliders along linear guides orthogonally arranged with respect to one another, this being in general set up by means of screws actuated by cranks and by motors.
The above-mentioned screws may form, in and of themselves the measuring means of the coordinates, as they may be connected to devices provided with electric and electronic circuits designed to deliver pulses during the screw movement. As an alternative, said coordinates may be measured by graduated rules and sliders provided with electric and electronic circuits as above.
When the images I.sub.1 and I.sub.2 are collimated, the operator sees a stereoscopic image of the object and a single collimation stereoscopic mark positioned on the object in correspondence with the point P. The coordinates x.sub.1 y.sub.1 and x.sub.2 y.sub.2 of I.sub.1 and I.sub.2 may thus be read on graduated rules of equivalent devices.
The motions required to locate the images I.sub.1 and I.sub.2 on the collimation optical system axis may be set up in different ways such as:
A. The motions may be made in two plate-holders in an independent manner.
B. A plate-holder can be assembled on a carriage provided with two motions x and y and, on this carriage, there may be assembled another carriage provided with two motions .DELTA.x and .DELTA.y which locate another plate-holder. The motion of the first plate-holder determines an equal motion of the second plate-holder which, however, may in its turn be moved in two directions which are orthogonal with respect to the former motion. Briefly, the two plate-holders are mechanically connected, but one of the two may undergo movements with respect to the other. A variation of this scheme, which does not allow superimposing the two carriages, is based on the use of differentials acting on a differential rim. One determines an equal motion of the two plate-holders. The other one, while acting on the satellite of the differential, determines the motion of one plate-holder with respect to the other.
c. The motions may be made on collimation optical units in independent manner.
d. The motions may be set up on the collimation optical units in a mechanically connected manner, corresponding to what was specified under (b) above for the plate-holders.
e. The motions may be divided. The plate-holders may move in an independent manner or mechanically connected in the direction x, and the collimation optical units may move in an independent manner or mechanically connected in the direction y. Alternatively, the plate-holders may be moved in an independent manner or mechanically connected in the direction y, and the collimation optical units may move in an independent manner or mechanically connected in the direction x.
f. The motions of the plate-holders and/or of the collimation optical units may also be determined in a polar manner; that is, instead of two motions in two orthogonal directions, the movement of a plate-holder and/or of a collimation optical unit may be set up by means of a rotation and a movement according to a direction set by the same rotation.
There is also known a type of stereocomparator in which instead of the four motions x.sub.1 y.sub.1 x.sub.2 y.sub.2, only three motions are possible, the fourth not being required owing to the particular shape of the collimation mark being employed.
In fact, if one uses a point-form collimation mark, one obtains the stereoscopic collimation only when both marks are brought into coincidence simultaneously on the images I.sub.1 and I.sub.2 of the point P, both in the direction x and in the direction y.
If one should use, on the contrary, a different and suitable form of mark, for instance, small segments or lines oriented according to the direction y, stereoscopic collimation would be possible also when there might not be a perfect coincidence of the images I.sub.1 and I.sub.2 of the point P with the two marks, but there would still remain a relatively small difference in the direction y, that is the operator acting on a stereocomparator having marks of said shape or form should carefully move the coordinates x.sub.1 y.sub.1 x.sub.2 to effect the collimation on the images I.sub.1 and I.sub.2 of the point P and coarsely adjust the coordinate y.sub.2, without however completely cancelling the parallax y (= y.sub.2 - y.sub.1 .noteq.0).
This operational mode produces however two draw-backs. Firstly, there is the fatigue of the operator's eye muscles. In fact, even if he does not positively perceive the residual parallax y owing to the particular shape of the marks used, to set up the stereoscopic collimation of the images I.sub.1 and I.sub.2 of the point P, he must rotate (unconsciously) one eye upwardly and the other downwardly, continuously varying the relative position.
This motion, within a short time, leads to a substantially fatigue of the muscles which move the eyes, this being a cause of headaches and thus making it impossible for an operator to continue stereoscopic collimation work for any substantial period of time.
Secondly, there is a decrease of collimation accuracy, the accuracy of the stereoscopic collimation of the two images I.sub.1 and I.sub.2 of the point P relying principally on the dimensions of the collimation mark. Said collimation accuracy is very good when the collimation mark has the form of a small well defined circular ball and decreases with the increasing of the dimensions thereof.
In particular, this stereoscopic collimation accuracy decreases substantially if the mark in the stereoscopic vision assumes the form of a small stick fluctuating in space and if some of its parts appear in relief (as the mark is complete both in the right hand and left hand optical paths) and other parts thereof do not appear in relief (as the mark is complete in one of the two optical paths but incomplete in the other one).